CBSE Class 10 Science 2020 · Paper 31/4/2 · Question 22

Light - Reflection and Refraction 3 marks Medium

(a)For the same angle of incidence of $4 5^{\circ}$ , the refraction angle in two transparent media P and Q is $2 0^{\circ}$ and $3 0^{\circ}$ respectively. Which of the two is optically denser and why ?
(b)Define 1 dioptre power of a lens.
(c)Find the focal length of a lens of power $+ 0.5$ D.

Answer

P is optically denser (smaller angle of refraction); 1 dioptre is the power of a lens of focal length 1 m; $f = + 2$ m.

(a) P is optically denser. For the same angle of incidence,

4 5^{\circ}

, the ray bends more towards the normal in P (

r = 2 0^{\circ}

) than in Q (

r = 3 0^{\circ}

); the smaller angle of refraction means a larger refractive index (

n_{P} \approx 2.07

n_{Q} \approx 1.41

), so light travels more slowly in P. (b) One dioptre is the power of a lens whose focal length is

1

metre. (c)

f = \frac{1}{P} = \frac{1}{+ 0.5 D} = + 2

m, that is

+ 200

cm: a converging (convex) lens.

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